The present invention relates to the technical field of combustion chambers for gas turbine engines such as turbojet engines. It is aimed in particular at a diffuser comprising a cowling on the combustion chamber.
In everything which follows, the terms “axial”, “radial” and “transverse” correspond respectively to an axial direction, a radial direction, and a transverse plane of the turbojet engine and the terms “upstream” and “downstream” correspond respectively to the direction in which the gases flow through the turbojet engine.
A conventional combustion chamber known as a divergent combustion chamber is illustrated in FIG. 10 which is an axial cross section showing half of the combustion chamber, the other half thereof being symmetric therewith respect to the axis (not depicted) of the engine. The combustion chamber 110 is contained in a diffuser 130 which is an annular space defined between an external casing 132 and an internal casing 134, into which a compressed oxidant originating upstream from a compressor (not depicted) is introduced via an annular diffuser duct 136.
This conventional combustion chamber known as a divergent combustion chamber 110 has an external wall 112 and an internal wall 114 which are coaxial and substantially conical, and which widen in the direction from upstream to downstream at a cone angle α1. The external 112 and internal 114 walls of the combustion chamber 110 are connected to one another toward the upstream end of the combustion chamber via a chamber end wall 116.
The chamber end wall 116 is provided with injection systems 118 through which injectors 120 which introduce fuel into the combustion chamber 110 in which combustion reactions occur pass.
These combustion reactions are intended to cause heat to radiate from the downstream to upstream direction toward the chamber end wall 116. In order to prevent damage to this chamber end wall 116 as a result of the heat, heat shields also known as deflectors 122 are provided, these being positioned on an interior face of the chamber end wall 116. They are cooled using jets of cooling air which enter the combustion chamber 110 through cooling orifices 124 pierced in the chamber end wall 116. These air jets, which flow in the direction from upstream to downstream, are guided by a chamber cowling 126, pass through the chamber end wall 116 through the cooling orifices 124 and impinge on an upstream face of the deflectors 122. The cowling 126 is also used to guide the air supplied to the injection systems 118. It has a substantially semi-toric shape and extends between two concentric edges for attachment to the edges of the chamber wall 116. A central portion of the cowling 126 is open to allow the fuel injection pipes to run as far as the injectors 120. The openings may be a substantially circular single slot. In this case, the cowling 126 is made up of two flanks known as fairings. As an alternative, the openings may consist of a collection of apertures each leading to a group of injectors.
In more recent designs of combustion chamber known as convergent combustion chambers, the external and internal walls of the combustion chamber are inclined such that they widen in the direction from downstream to upstream rather than from upstream to downstream as was the case in the “divergent” conventional combustion chambers described hereinabove.
A “convergent” combustion chamber 10 such as this is illustrated in part in FIG. 11, in axial section. This FIG. 10 shows an axial direction 100 parallel to the axis of the turbojet engine, a generatrix direction 200 of the combustion chamber 10, and a cone angle α2 between these two axes 100, 200. The combustion chamber 10 comprises an external wall 12 and an internal wall 14 which are coaxial and substantially frustoconical, and which widen in the direction from downstream to upstream at a cone angle α2.
The external 12 and internal 14 walls of the combustion chamber 10 are connected to one another toward the upstream end of the combustion chamber by a chamber end wall 16 which is a substantially frustoconical part running between two substantially transverse planes and widening in the direction from upstream to downstream. The chamber end wall 16 is connected to each of the two, external 12 and internal 14, walls of the combustion chamber 10. It is provided with injection systems 18 through which injectors 20 pass these passing through the outer casing 32 and introducing fuel into the combustion chamber 10 where the combustion reactions take place.
The combustion chamber 10 is contained in a diffuser 30 which is an annular space defined between an external casing 32 and an internal casing 34 and into which a compressed oxidant originating upstream from a centrifugal compressor (not depicted) is introduced via an annular diffuser duct 36. The oxidant is generally air. The combustion chamber 10 is positioned right into the diffuser 30 between an external part 28 and an internal part 29 of this diffuser 30. The external part 28 of the diffuser 30 constitutes an annular and conical space contained between the external casing 32 and the external wall 12 of the combustion chamber 10. The internal part 29 of the diffuser 30 constitutes an annular and conical shape contained between the internal casing 34 and the internal wall 14 of the combustion chamber 10.
Some of the oxidant, generally air, enters the diffuser 30 followed by the combustion chamber 10 to participate in the combustion reactions taking place therein. The entry of oxidant to the combustion chamber 10 is guided by the cowling 226. Some more of the oxidant flows into the diffuser 30, bypassing the combustion chamber 10, on the one hand through an external part 28 of the diffuser 30 which is contained between the external casing 32 and the external wall 12 of the combustion chamber and, on the other hand, through an internal part 29 of the diffuser 30 which is contained between the internal casing 14 and the internal wall 34 of the combustion chamber.
With a configuration such as this, an imbalance arising between the bypass flow bypassing the combustion chamber 10 around the outside, in the external part 28 of the diffuser 30, and the bypass flow bypassing this same combustion chamber 10 on the inside, through the internal part 29 of the diffuser 30. It then follows that the pressure drops available across the external wall 12, and which correspond to the difference in pressure between the external part 28 of the diffuser 30 and the inside of the combustion chamber 10 exceed the pressure drops available across the internal wall 14, which correspond to the difference in pressure between the internal part 29 of the diffuser 30 and the inside of the combustion chamber 10.
This imbalance in the pressure drops between the external wall 12 and the internal wall 14 is detrimental to the correct operation of the combustion chamber 10 because the primary jets enter and are diluted better in the region of the external wall 12 than in the region of the internal wall 14. Furthermore, because the pressure drops available are lower across the internal wall 14, this wall is more difficult to cool.
What is more, the pressure drops available for supplying air to the injection systems 18 is reduced because the diffuser duct 36 does not lie directly facing the injection systems 18.